This invention relates to pressurized vessels having rotating internal components. In particular, this invention relates to a rotating shaft to drive a stirrer in a pressurized agitator and digester vessels.
The pulp and paper industry, as well as other process industries, employ chemical reactions in processes that are often performed under pressures greater than atmospheric pressure. Typically, these processes are performed within vessels that maintain the product at predetermined super-atmospheric pressures and at elevated temperatures that promote the desired chemical reaction. A continuous or batch pulp digester vessel are examples of vessels within which are performed chemical reactions under elevated pressures and temperatures.
The processes being performed often require agitation or stirring of the product in the vessel during the chemical reaction and while the product is under pressure and at elevated temperatures. The agitation is typically effected by a shaft-driven agitator. An electric motor drives the shaft via a power transfer device, such as a transmission, that may include belts, drive chains and a gear reducer.
The drive shaft of the agitator penetrates the wall of the pressure vessel. Bearings support the drive shaft. The bearings are mounted in an outlet housing below the digester vessel. The bearings reduce the friction between the rotating or reciprocating shaft and the support housing. Typically, the bearings are roller bearings, such as spherical and cylindrical anti-friction bearings, or journal bearings that are self-lubricating or have reduced-friction properties. Conventionally, two bearings are arranged along a length of the drive shaft. Both bearings are above the power transfer devices which engages a bottom end of the drive shaft. The two bearings generally include a thrust-radial bearing and a radial bearing. The thrust-radial bearing supports axial loads applied to the drive shaft by the digester. The thrust-bearing prevents substantial axial forces from being applied power transfer device coupled to the drive shaft. The thrust-bearing and radial bearing support the shaft with respect to radial forces, isolate the power transfer device from radial and axial loads applied by the digester to the shaft, and prevent the shaft from wobbling during rotation.
The drive shaft is intentionally relatively long to accommodate the two bearings and to prevent shaft wobbling due to force moments resulting from the application of radial forces. The bearings are conventionally separated by a substantial distance, such as two to three feet. The separation distance requires the length of the drive shaft to be relatively long below the digester. The digester must be sufficiently elevated to accommodate the long drive shaft and the power transfer device. In certain digester applications, it is difficult to elevate the digester vessel sufficiently to accommodate a long drive shaft and the power transfer device, e.g., a gear box transmission. Sufficient ground clearance between the digester vessel and the outlet housing is also needed to install an extended outlet housing and associated drive shaft where the outlet housing is extended to include a conduit for wash liquor flowing into the vessel. For at least applications where ground clearance of the vessel is a concern, there is a need for an outlet housing having a short drive shaft and, preferably, an associated short transmission.